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I read some posts, like this one, saying it this not possible to back feed a RPI from the USB sockets.

But I think this is possible. I have two evidences for that:

  1. This post from Eben Upton (Raspberry CEO) where he wrote « It is now possible to reliably power the RPI from a USB hub that back feeds power… »
  2. The schemas here. Excepted for revision 1.0, I can’t see any diode or fuse preventing the current coming from the USB sockets to fully power the RPI.

So, am I missing something?

Community
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Jim
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1 Answers1

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It depends on what you mean by a modern Pi.

You can not (see later for the proviso) usefully backpower a Pi with the 40 pin expansion header from the USB ports.

The Eben Upton post is from late 2012 and is referring to revision 2 of the original Pi B.

On the Pis with the 40 pin expansion header the USB ports are enabled in software. Until they are enabled they can not backpower the Pi.

So to boot a Pi with the 40 pin expansion header you need to supply power via the microUSB or via the 5V pin on the expansion header. Once it is booted and the USB sockets are enabled you can then remove the original power source and backpower. That is why I say you can not usefully backpower.

joan
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  • I agree it is not useful. Have you tried this? AFAIK, from studying the circuitry backpowering will not work on B+ and Pi2. I admit I have not tried; I used to great lengths to prevent my model B from being backpowered. – Milliways Sep 03 '15 at 23:56
  • @Milliways I would have tried at the time as the possibility of back powering the B+, Pi2 was against conventional wisdom. I would have tried just now but can't find the power supply for the hub. There is at least one report that the A+ can be backpowered without first "booting" with a conventional power supply. – joan Sep 04 '15 at 07:49